1. Field of the Invention
The present invention relates to an explosive composition containing micro-voids, and more particularly relates to an explosive composition containing specifically limited hollow microspheres as micro-voids and having an improved low temperature detonability represented by the lowest detonation temperature after lapse of a long period of time in a small diameter cartridge having a diameter of 25 mm.
2. Description of the Prior Art
There have been used various kinds of micro-voids in industrial explosives in order to lower the density of an explosive and to improve the detonation properties, such as detonation sensitivity, propagation property of detonation, and the like.
The term "micro-voids" means hollow microspheres, bubbles formed from a foaming agent, bubbles formed by blowing mechanically (physically) air into the raw material mixture of the explosive, and the like.
Explosives containing bubbles have a drawback that the bubbles leak gradually from the explosives during a storage for a long period of time and the detonation properties of the explosives become poor.
While, explosives containing hollow microspheres are seldom in the leakage of gas contained in the microspheres from the explosives during the storage, but have the following drawbacks.
That is, when inorganic hollow microspheres, such as glass, volcanic ash and the like, are intended to be used as hollow microspheres, a large amount of inorganic hollow microspheres must be used in order to produce an explosive having a low specific gravity, because the density of the inorganic hollow microspheres is higher than 0.1 g/cc due to their inherent property and to the thickness of the shell wall. Accordingly, the use of inorganic hollow microspheres is disadvantageous from the viewpoint of the strength of the resulting explosive due to the quite inertness of the microspheres at the explosion and further is disadvantageous from the viewpoint of the cost of the microspheres. Furthermore, many kinds of inorganic hollow microspheres have a high mechanical strength and are hardly broken under a low pressure, and therefore an explosive containing inorganic hollow microspheres is small in the amount of heat to be supplied to the explosive by the adiabatic compression of gas, which is contained in the interior of the inorganic hollow microspheres and acts as an initiator for the detonation of the explosive, and hence the explosive is low in the detonation sensitivity.
In order to obviate these drawbacks, there has been attempted to improve the detonation property of an explosive without deteriorating its strength by containing hollow microspheres made of resin (hereinafter, referred to as resin hollow microspheres) in the explosive, which resin acts as a combustible material at the explosion of the explosive.
As the resin hollow microspheres, there are known thermoplastic resin hollow microspheres and thermosetting resin hollow microspheres.
For example, there have been disclosed in U.S. Pat. No. 3,773,573 that dynamite, gelatinized nitromethane explosive, cast explosive and slurry explosive, which contain 0.1-2% by weight of thermoplastic resin hollow microspheres consisting of a vinylidene chloride-acrylonitrile-methyl methacrylate terpolymer resin (hereinafter referred to as Saran (registered trademark by Dow Chemical Co.)) and having a density of 0.032 g/cc and a particle size of 5-100 .mu.m, are superior to those containing inorganic hollow microspheres or phenolic thermosetting resin hollow microspheres in the strength due to the reactivity of the terpolymer resin.
Further, there has been proposed, in U.S. Pat. No. 4,110,134, a water-in-oil emulsion explosive, which contains 0.24-1% by weight of Saran thermoplastic resin hollow microspheres or phenolic thermosetting resin hollow microspheres, each hollow microspheres having a density of 0.032 g/cc and an average particle size of 30 .mu.m, and can be detonated after a lapse of one year or more in a small diameter cartridge (1.25 inch diameter) by means of a No. 6 blasting cap without containing explosive sensitizer and detonation catalyst.
However, these resin hollow microspheres have a shell wall consisting of a single layer formed of thermoplastic resin or thermosetting resin, and have the following drawbacks.
The thermoplastic resin hollow microspheres have a low density, that is, have a thin shell wall, and hence they are apt to be easily compressed even under a relatively low pressure and are often broken during the production of explosive. Moreover, the thermoplastic resin has a softening point, and therefore when some kinds of explosives, for example, water-gel explosive, which are required to be mixed with hollow microspheres at high temperature in the production thereof, are produced, the thermoplastic resin hollow microspheres are easily broken due to the heat and pressure, to which the explosives are exposed during the production.
When a part of hollow microspheres are broken during the production of an explosive containing the hollow microspheres, gas contained therein leaks during storage of the explosive for a long period of time, and as a result the explosive has not a satisfactorily high detonation sensitivity at low temperature after lapse of a long storage time. Further, an explosive containing hollow microspheres, a part of which have been broken during the production of the explosive, is poor in the resistance against shock from pre-explosion in an adjacent bore hole at the blasting face, and hence the explosive is apt to be misfired and remain, and is disadvantageous in view of the safety maintenance.
While the thermosetting resin hollow microspheres have a relatively high density, and hence they have the same drawbacks as those of the above described inorganic hollow microspheres. That is, in order to produce an explosive having a low specific gravity, a relatively large amount of at least 1% by weight of thermosetting resin hollow microspheres must be contained in the explosive. As the result, the oxygen balance of the explosive is apt to be negative, and the after-detonation fume thereof is apt to be ill due to uncomplete detonation. Furthermore, the thermosetting resin hollow microspheres are hardly broken due to their relatively thick shell wall, and therefore an explosive containing the thermosetting resin hollow microspheres is small in the amount of heat to be supplied to the explosive by the adiabatic compression of gas, which is contained in the interior of the thermosetting resin hollow microspheres and acts as an initiator for the detonation of the explosive, and hence the explosive is low in the detonation sensitivity.